dt-bindings:iio:accel:adis16201 and adis16209 bindings

These two devices have different internal characteristics, but their
external connectivity and as a result device tree descriptions are
identical.

Note that neither driver in Linux currently has an of_match_table
but instead rely on matching via name alone.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210401174112.320497-2-jic23@kernel.org